In the oil and gas industry, seismic prospecting techniques commonly are used to aid in the search for and evaluation of subterranean hydrocarbon deposits. An exemplary seismic prospecting operation includes three stages: data acquisition, data processing, and data interpretation, and the success of the operation depends on satisfactory completion of the three stages. In the data acquisition stage, a seismic source is used to generate an acoustic signal that propagates into the earth and is at least partially reflected by subsurface seismic reflectors. The reflected signals are detected and recorded by an array of seismic receivers located at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes. During the data processing stage, the recorded seismic signals, e.g., seismic amplitude response, are refined and enhanced using a variety of procedures that depend on the nature of the geologic structure being investigated and on the characteristics of the raw data. In general, the purpose of the data processing stage is to produce an image of the subsurface from the recorded seismic data for use during the data interpretation stage. The purpose of the data interpretation stage is to determine information about the subsurface geology of the earth from the processed seismic data. The results of the data interpretation stage may be used to determine the general geologic structure of a subsurface region, or to locate potential hydrocarbon reservoirs, or to guide the development of an already discovered reservoir.
The seismic amplitude response of a subsurface region containing sands is dependent upon both the rock properties and the pore fluid properties. Fluid prediction via amplitude versus offset (AVO) analysis is commonly used for risk reduction in the exploration, development and production of hydrocarbon resources. AVO analysis recognizes that the seismic reflection amplitude may vary with change in distance between the seismic source and receiver, where the offset is the distance between the seismic source and the receiver. Specifically, the variation in seismic reflection amplitude can be indicative of differences in lithology and fluid content in the subsurface rock layers. For example, AVO analysis may be used to determine thickness, porosity, density, velocity, lithology and fluid content of rock layers. A seismic reflection amplitude from a low-impedance, hydrocarbon-bearing sand typically increases with increasing offset distance. However, various rocks and fluids have different reflection amplitudes versus offsets indicative of the specific rock and fluid composition, e.g., various increasing amplitude with offset or decreasing amplitude with offset depending on the type of rock or fluid present. The accurate seismic prediction of fluids is used in resource exploration and can be used as a tool to aid in understanding the lithology and porosity of the rock.
Intercept-gradient crossplots derived from near and far angle-stack data allow for the identification of hydrocarbons that are separated from the wet sand or background shale response on seismic data. Crossplotting is a fundamental process for AVO analysis and has been widely documented to predict reservoir quality, including porosity, lithology, and fluid density. For example, one or more exemplary techniques of the background art which utilize crossplotting of seismic attributes to understand seismic response to hydrocarbons are further described in “A Comparison of Hydrocarbon Indicators Derived from AVO Analysis,” SEG Expanded Abstracts 26, 279, by Feng et al., (2007); and in “Is There a Basis for All AVO Attributes?,” SEG Expanded Abstracts 26, 244, by Zhou et al. (2007); “Quantitative AVO Analysis,” SEG Expanded Abstracts 24, 273, by Kelly et al. (2005).
In a clastic regime, thick sands with a strong AVO response are generally easily identified using intercept-gradient crossplotting. However, the present inventors have determined that the vertical edges of thick sands typically display overlap in intercept-gradient space with wet sands or shales, and are consequently not identified using this crossplotting technique.